1. Field of the Invention
The use of non-formaldehyde reducing agents of the hypophosphite type provide substantial advantage over some of the more conventional reducing agents, such as formaldehyde and the boro compounds, heretofore used almost exclusively in electroless copper plating. The use of hypophosphite type reducing agents in electroless copper solutions heretofore has, however, been restricted in practical commercial plating operations to baths of pH values generally well in excess of pH 5.0. This invention relates to improvements in electroless copper plating baths employing hypophosphite reducing agents, but having substantially lower pH operating ranges.
2. Description of the Prior Art
The predominant commercial practice in electroless deposition of copper, prior to the recent introduction of the hypophosphite electroless copper baths mentioned above, uses highly alkaline, complexed cupric solutions containing formaldehyde as the sole reducer. The volatility of formaldehyde, and its tendency to undergo the well-known, wasteful, Cannizzaro side reaction, and the long-term relative instability of formaldehyde-bearing copper plating solutions, have prompted the art to search for non-formaldehyde plating solutions.
U.S. patent application Ser. No. 909,209 filed May 25, 1978, now U.S. Pat. No. 4,209,331 provides one of the first disclosures enabling commercially practical use of hypophosphite reducing agents in electrolessly plating copper onto non-conductors. That application deals extensively with what prior art there is relating to hypophosphite-type electroless copper plating solutions, and that disclosure is incorporated herein by reference. Essentially, it was shown that while hypophosphite has gained industry-wide acceptance for depositing nickel in electroless plating operations, there were no commercial processes being successfully used to the inventors' knowledge in which hypophosphite agents were employed to electrolessly plate copper. It is disclosed in that application that by maintaining a specified complexer/pH relationship in the copper bath at pH values of 5.0 to 13.0, plating of copper with hypophosphite reducing agents is practical, and that the system has advantages for commercial operations.
It is taught in application Ser. No. 909,209, now U.S. Pat. No. 4,209,331 however, that at pH values below 5 copper deposition is virtually absent or is of little usefulness because the deposit has poor conductivity, is powdery, or both. Another troublesome complication that led to the foregoing conclusion is the appearance of a precipitate when the bath is heated to temperatures in the range of 140.degree. to 160.degree. F. Operation of a bath at this temperature is desired to minimize voids in the copper film deposited. It was previously believed that because of the ineffectiveness of most commercial organic chelating agents to strongly coordinate copper ions in solution at pH values below 5, there was little chance to overcome the difficulties of poor coating conductivity and low solution stability. In the aforesaid application, numerous examples document the basis for previously reaching that conclusion, the following being representative:
Table A Ex. 9, 10, 11, 12 PA1 Table B Ex. 30, 31, 32, 33 PA1 Table C Ex. 51, 52, 53, 54 PA1 Table D Ex. 64, 65, 66, 67
To the extent that applicant can find in the prior art any reference to hypophosphite-based, low pH (5 or less) electroless copper plating baths, such baths do not contain chelating agents. A case in point is U.S. Pat. No. 3,046,159 disclosing compositions of cupric oxide, ammonium sulfate or chloride, and sodium hypophosphite. The operating pH of the bath is given as 3.0; the operating temperature is given as 93.degree. C. (200.degree. F.). However it is found in practicing the invention in accordance with the teaching of the patent that the deposits formed are powdery, have poor adhesion and are poorly conducting, and burn off if attempt is subsequently made to build them up to useful thicknesses by electroplating techniques.
In another reference, IBM Technical Disclosure Bulletin, Vol. 12, No. 2, dated July 1969, it is confirmed that a bath quite similar to the one described in U.S. Pat. No. 3,046,159 produces a deposited film "the conductivity of which is not very high." The bulletin goes on to recommend that in order to produce a conductive layer, "the plating is continued in an alkaline electroless copper bath;" i.e. a conventional formaldehyde containing bath.
In another reference, U.S. Pat. No. 3,650,777, fleeting reference is made to possible use of hypophosphite in acidic electroless copper plating baths (see Column 2, lines 15-17) without, however, teaching how this is accomplished and without giving a single example of a bath to illustrate any reduction to practice.
In summary, therefore, the foregoing patents and technical literature typically illustrate prior art teaching in respect to the difficulties of attempting to use hypophosphite as a reducing agent for electroless copper plating at low pH value, and the conclusion advanced has been that the principal difficulties are poorly conductive powdery copper films and bulk-precipitation of insoluble particles, which make such a bath inoperative for practical applications.